It isn't that straight forward, but works okay. Can you just set sources at boundaries using Fluent menus? You can just use the SMASEX(L,i) variable for the source of species i in cell L and let Fluent take care of the rest. Good Luck.

For L = first active cell to last active cell, set SMASEX for species i to source value (in kg/s). This can be done in any active user subroutine (that has #INCLUDE statement for correct variable). Make sure mass balance balances (i.e. using Total zone integral in View alpha). It may take one more tweak.